Method for supplying washing liquid to a process for cooking cellulose pulp

ABSTRACT

The invention relates to a method for supplying washing liquid to a digester ( 1 ) for cellulose pulp and a subsequent diffuser washing device ( 2 a,  2 b). A high, disturbance-free production capacity can be obtained by using a common feed pump (P 1 ) which, via a main feed line ( 10 ), supplies both the diffuser washing device and the digester with washing liquid in parallel. The first pump (P 1 ) establishes a pressure on the washing liquid which is in the interval 2–6 bar, and is preferably 4 bar, while the second pressure level, which is established by the diffuser pump device (P 2   a , P 2   b ), is in the interval 6–12 bar, but is at least 2 bar higher than the first pressure level, and the third pressure level, which is established by the digester pump (P 3 ), is in the interval 12–25 bar The system becomes self-regulating in the event of any operational disturbances in the flow of washing liquid and, if the pumps are appropriate, a countercurrent flow of washing liquid through the system is essentially maintained.

PRIOR APPLICATION

This application is a U.S. national phase application based onInternational Application No. PCT/SE02/00832, filed 29 Apr. 2002,claiming priority from Swedish Patent Application No. 0101557-7, filed 2May 2001.

The present invention relates to a method for supplying washing liquidto a digester for cellulose pulp and a subsequent diffuser washer.

STATE OF THE ART

The combination of a continuous digester and a subsequent diffuser isfrequently employed when manufacturing chemical cellulose pulp,especially kraft pulp. As is customary, washing liquids for thedifferent constituent processes are conducted in countercurrent to theflow of the pulp, with the washing liquid first being fed to thediffuser, after which the wash filtrate which is obtained from thediffuser is conducted to the washing zone of the digester as dilutionliquid and/or washing liquid. This countercurrent flow of washingliquids is implemented for the purpose of reducing, to the greatestpossible extent, the consumption of fresh water, something which hasbeen regarded as being an absolute necessity for being able to close theprocess and minimize the discharge of process water from the process.

U.S. Pat. No. 4,123,318 discloses such a countercurrent conveyance ofwashing liquid, with filtrate from a washing stage subsequent to thedigester being conducted to the preceding washing stage in the processand from the washing stage to the washing zone of the precedingdigester.

U.S. Pat. No. 5,066,362 also discloses a system in which the washingliquid is firstly conducted to a diffuser after which the wash filtratewhich is obtained from the diffuser is conducted to the washing zone ofthe digester. This provides a gradual leaching-out/washing of the pulpand a gradually increasing content of released material in the washfiltrate which is being conducted in countercurrent to the flow of thepulp. The washing liquid is also conducted strictly in countercurrent tothe flow of the pulp in SE,C,501848, with the washing liquid firstlybeing conducted to a diffuser, after which the diffuser filtrate isconducted to the washing zone of the digester. All that otherwise takesplace is that a supplement of white liquor is conveyed to the diffuserfiltrate for the purpose of increasing the level of alkali in thediffuser filtrate.

In order to increase the operability in these systems, buffer tanks orintermediate storage tanks are used for the diffuser filtrate so as toensure that washing liquid can be supplied to the digester at a constantrate. These tanks frequently have to be designed as expensive pressurevessels since the temperature of the washing liquids at these positionsis close to, or above, the boiling point of the washing liquid.

The known systems function satisfactorily when production is moderateand if there are no disturbances in the process. However, if thediffuser, whose washing efficiency is normally very good, should exhibita tendency to cause clogging in the flow of washing liquids, due, forexample, to inappropriate handling, this then affects the digesterdirectly as a result of a corresponding decline in washing liquid anddilution liquid being supplied to the bottom of the digester. While thisgreatly reduces the washing effect at the bottom of the digester,another marked effect is that it can become difficult to achieve thedilution in pulp concentration which is required in association with thepulp being blown from the digester, something which immediately involvesthe risk of giving rise to clogging in the flow of pulp from thedigester and of the temperature of the pulp rising rapidly to anundesirable level. In order to deal with this clogging, it may benecessary for the digester to be shut down completely.

OBJECT AND PURPOSE OF THE INVENTION

The principal object of the invention is to avoid the disadvantages ofthe prior art and to enable the operability of the cooking process to beimproved in the digester/diffuser constituent system, with it beingpossible to increase the productivity in an existing digester (more pulpper 24 hours) by more than 100%. By means of totally integrating thedigester and the diffuser, most of the extraction from the cooking caninstead be transferred to the diffuser and the loading on the digester'sextraction screens can consequently be reduced, thereby making itpossible to achieve this substantial upgrading of the digester capacity.This becomes particularly advantageous when upgrading existingdigesters. As a result of a large part of the extraction from thecooking being made in the pressure diffuser, the pressure and/or liquorlevel in the digester can be controlled by means of this extraction fromthe diffuser.

Another object is to make the process less sensitive to disturbances inthe flow of washing liquid through the diffusers, with it being possibleto maintain the digester process under optimum conditions despite theoccurrence of momentary disturbances in the flow of washing liquidthrough a diffuser.

Yet another object is to optimize the procedure with regard to supplyingwashing liquid such that only that pressure level which is required ismaintained at each point in the supply system, thereby making itpossible to set the greater part of the system at a moderately lowlevel, thus enabling thinner pipelines to be used.

Yet another object is to obtain a system for providing the diffuser anddigester with washing liquid, which system is self-regulating and can atany moment guarantee the requisite flow of washing liquid to the washingand dilution zone of the digester. Pulp can then be guaranteed to be fedout from the digester at the correct consistency and temperature,thereby actively counteracting any tendency towards clogging in the pulpflow leaving the digester, on the one hand, and, on the other hand,eliminating any lowering in the quality of the pulp due to the blowingtemperature being too high. As a result of this self-regulation, themajor part of the flow of washing liquid to the bottom of the digesterwill pass via the diffusers during normal operation. However, as soon asany local blockage of washing liquid occurs in a diffuser, aself-regulation then takes place, with this self-regulation compensatingfor the decline in filtrate flow from the diffuser with an equivalentflow from the diffuser's washing liquid tank.

Yet another object is to be able to remove all the buffer orintermediate storage tanks for the diffuser filtrate, therebysubstantially reducing the investment costs.

LIST OF DRAWINGS

FIG. 1 shows a diagram of a system according to the invention, with adigester and two subsequent diffusers which are coupled in parallel:

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a system according to the invention for supplying washingliquid to a continuous digester 1 for cellulose pulp and a subsequentdiffuser washing device 2 a, 2 b in which washing liquid 12 a, 12 b forthe diffuser washing device is drawn from a storage tank 4 and in whichwashing liquid 17 is conducted to the bottom of the digester in order toobtain a dilution and/or a displacement washing at the bottom of thedigester.

The cellulose pulp which is dissolved in the digester is fed out in theflow 20 and a Y-coupling arrangement is preferably used to ensure theestablishment of two constituent flows 20 a, 20 b to two diffusers,which consequently operate in parallel. The central branch in the Ycoupling provides an alternative auxiliary feeding route for the flow 20a or else the flow 2Ob. After the diffusers, washed pulp 21 a, 21 b iswithdrawn from each respective diffuser and combined in the flow 22 forsubsequent conduction to a storage tank 3 for the cooked and washedpulp.

The diffuser is a conventional pressure diffuser which maintains thepressure in the pulp during the washing. The diffuser can be either fedfrom the bottom or the top, and the figure shows a top-fed pressurediffuser. The pulp is conducted into the top of the pressure diffuser,after which it passes down through the pressure diffuser in a thincircular layer, with the washing liquid being fed in from outside thelayer (marked grey in the figure) by way of a multiplicity ofdistribution nozzles 14 a, 14 b, which are arranged, on the one hand,around the whole of the pressure diffuser (not shown) and, on the otherhand, over the greater part of the height of the pressure diffuser (asshown in the figure). The washing filtrate which is obtained after thewashing liquid has passed through the fibre layer is extracted from theinside of the pressure diffuser, 15 a, 15 b, with these filtrate flowsbeing indicated by 13 a and 13 b in the figure.

That which is characteristic of the invention is that the washing liquidfrom the storage tank 4 is pressurized to a first pressure level, by wayof a first feeding pump P1, and is subsequently conducted to a main feedline 10. At a first upstream point 10 a in the main feed line, a firstconstituent flow 11 is drawn from this main feed line 10 to a diffuserpump device P2 a/P2 b which pressurizes the washing liquid to a secondpressure level and conducts it onward, 12 a, 12 b, at this secondpressure level to the washing liquid inlets 14 a, 14 b belonging to adiffuser washer. The wash filtrate 13 a, 13 b from the washing liquidoutlet 15 a, 15 b in the diffuser washer is conducted back to the mainfeed line 10 at a second point 10 b in the main feed line, with thissecond point being located downstream of the first upstream point 10 ain the main feed line. A combined flow of washing liquid from thestorage tank 4 and wash filtrate 16 from the diffuser washing device 2a, 2 b is conducted, via the main feed line, to a digester pump P3,which pressurizes the combined flow to a third pressure level, and isconducted onwards 17, at this third pressure level, to washing liquidnozzles 18 which are arranged at the bottom of the digester.

The pressure levels are related to each other such that the firstpressure level is lower than the second pressure level, which secondpressure level is in turn lower than the third pressure level, with eachrespective pressure level constituting an increase in pressure of atleast 50–125% relative to the next lower pressure level.

In a preferred application, this corresponds to the first pressurelevel, which is established by the first feed pump P1, being in theinterval 2–6 bar, preferably 4 bar, to the second pressure level, whichis established by the diffuser pump device P2 a, P2 b, being in theinterval 6–12 bar, but at least 2 bar above the first pressure level,and to the third pressure level, which is established by the digesterpump P3, being in the interval 15–25 bar.

In a conventional pressure diffuser, the pump P2 a should deliver apressure which guarantees flow through the diffuser. The following fallsin pressure occur in the diffuser circuit:

a) static height (difference in height between the pump and the upperpart of the diffuser, approx. 2.5 bar);

b) pressure fall in lines and control valves (FC etc., approx. 1 bar);

c) the diffuser's inlet nozzles (14 a, approx. 2 bar);

d) pressure fall through the pulp bed (approx. 0.5 bar);

e) prevailing pulp pressure at the top of the diffuser (depends on thedigester pressure, its height + pressurization level, can typically be 6bar).

With a feed pump P1 which delivers 4 bar, a guaranteed flow under normaloperation is obtained if P2 a provides an increase in pressure of a good8 bar, on the assumption that the pulp pressure at the top of thediffuser is 6 bar. The governing prerequisite is that, under normaloperation, the pressure on the flow 16 should at least be equal to thepressure of the feed pump.

The connection points 10 a and 10 b should expediently be located on themain feed line such that the point 10 b lies at a distance from thedigester pump P3 which is within 10% of the entire length of the feedline 10 and such that the point 10 a lies at a distance from the outletof the feed pump P1 which is within 10% of the entire length of the feedline 10. During normal, disturbance-free operation in the system, thefall in pressure in the feed line 10 between the points 10 a and 10 bshould expediently exceed or correspond to the increased pressure in thewashing liquid filtrate 13 a, 13 b.

In the embodiment shown, the diffuser washing device consists of twodiffusers which are coupled in parallel as regards the receipt andtreatment of cooked pulp obtained from the digester. This configurationprovides increased availability in the system, with it being possible toredirect part of the pulp flow between diffusers depending on anypossible momentary disturbances in the system.

The diffuser pump arrangement consists of two diffuser pumps P2 a and P2b, which are coupled in parallel and which, on the inlet side, areconnected to the same supply flow 11, which is taken from the main feedline 10, with each respective outlet from each of the diffuser pumpsonly supplying one diffuser with its respective washing liquid flow 12a, 12 b. While P2 a and P2 b can, of course, be replaced with a singlepump, two separate pumps provide increased availability. The washfiltrates 13 a and 13 b, which are obtained from each respectivediffuser, are first of all combined to form a joint flow 16 in a Ycoupling,. after which this combined wash filtrate flow 16 is returnedto the main feed line at a point 10 b on the main feed line closer tothe inlet to the digester pump P3.

The whole of the system becomes self-adjusting with regard to anypossible variations in the pulp flow and/or washing liquid flows, and nobuffer tanks or intermediate storage tanks are required. When the pumpsare adequate, the main flow of the washing liquid filtrate develops inthe circuit 4-P1-11-P2 a/P2 b-2 a/2 b-16-P3-1; however, as soon asmomentary restrictions in flow occur in the circuit due to pressurefalls in the diffuser which are too great, the decline in the flow isthen compensated by flow in the circuit P1-10 a-10-10 b-P3. In anapplication in which the capacity in an existing digester was increasedfrom 1 500 AdT/24 hours to somewhat more than double the capacity, i.e.3 200 Adt/24 hours, this then means that the rate of flow of the chipsin the digester was more than doubled, representing a dramatic increasein production capacity. In connection with flow rates of this magnitudein the system, great demands are made on the retention of a high levelof supply of washing liquid and dilution liquid to the bottom of thedigester, thereby guaranteeing correct dilution and washing prior to thepulp being blown from the digester. The invention allows the existingproduction capacity to be increased dramatically while at the same timemaintaining a high degree of operability and availability.

The invention can be varied in a number of ways within the scope of theattached patent claims.

For example, the parallel diffusers can be replaced by a single diffuseror, alternatively, supplemented with a third diffuser which is coupledin parallel.

The embodiment depicted in FIG. 1 shows flow-controlling valves FC whichact on flows 12 a and 12 b and also on the flow 17. The flow 17 isnormally divided up into several constituent flows which are added tothe bottom of the digester at different heights.

The diffuser/diffusers 2 a, 2 b can also have one or more diffuserswhich are coupled in series, for example such that the pulp flow eitherpasses by way of 2 a ₁-2 a ₂-2 a ₃ or, alternatively, by way of 2 b ₁-2b ₂-2 b ₃, where the subscript ₁/₂/₃ indicates the order in thedirection of flow of the pulp. A first joint feed pump can then provideall the diffusers in at least one main feed line with pressurizedwashing liquid from a diffuser tank, which branch is in opencommunication with washing/dilution nozzles in the bottom of thedigester, with the wash filtrate from each respective diffuser beingreturned to the main feed line. The pump P2 ₃ which supplies the lastdiffuser is designed/selected such that the increase in pressurerelative to the pressure in the main feed line exceeds the pressure fallin the diffuser and the prevailing pulp pressure at that position. Whenthe diffusers are identical, a gradual increase in pressure in thedirection P2 ₃→P2 ₂→P2 ₁, on a par with 2 bar, should normally beimplemented. In a typical example in which the first feed pump provides4 bar, the pressure levels P2 ₃/P2 ₂/P2 ₁ can be on a par with ₃/₅/₇ barif the prevailing pulp pressure in each respective diffuser (given bythe pressure in the preceding digester) is on a par with ₄/₆/₈ bar.

Sometimes, use is made of two or more parallel pumps which replace thepump P3, with each of the respective pumps supplying its own constituentflow to the bottom of the digester. Flow-controlling valves FC are alsoarranged in the Y coupling which regulates the flow of pulp to theparallel diffusers. Pressure-controlling outlet valves PC are alsoarranged in the outlets.

1. Method for supplying washing liquid to a digester (1) for cellulosepulp and a subsequent diffuser washing device (2 a, 2 b), with washingliquid for the diffuser wash being collected from a storage tank (4) andwith washing liquid being conducted to the bottom of the digester forthe purpose of obtaining a dilution and/or displacement wash at thebottom of the digester, characterized in that the washing liquid fromthe storage tank (4) is pressurized to a first pressure level via afirst feed pump (P1) and conducted onwards to a main feed line (10), afirst constituent flow (11) is drawn from the main feed line (10), at afirst upstream point 10 a) on the main feed line, to a diffuser pumpdevice (P2 a, P2 b), which pressurizes the washing liquid to a secondpressure level, and is conducted onwards (12 a, 12 b), at this secondpressure level, to washing liquid inlets (14 a, 14 b) of a diffuserwashing device, wash filtrate from a washing liquid outlet (15 a, 15 b)of the diffuser washing device (2 a, 2 b) is returned to the main feedline (10) at a second point (10 b) on the main feed line, which point islocated downstream of the first upstream point (10 a) on the main feedline, a combined flow of washing liquid from the storage tank (4) andwash filtrate (16) from the diffuser washing device (2 a, 2 b) isconducted, via the main feed line (10), to a digester pump (P3) whichpressurizes the combined flow to a third pressure level and conducts thecombined flow (17) onwards, at this third pressure level, to washingliquid nozzles (18) which are arranged at the bottom of the digester. 2.Method according to claim 1, characterized in that the first pressurelevel is lower than the second pressure level, which second pressurelevel is in turn lower than the third pressure level.
 3. Methodaccording to claim 2, characterized in that each respective pressurelevel constitutes an increase in pressure of at least 50–125% ascompared with the next lower pressure level.
 4. Method according toclaim 1, characterized in that the diffuser washing device (2 a, 2 b)consists of at least one pressure diffuser, which maintains a positivepressure in the pulp during the washing.
 5. Method according to claim 1,characterized in that the diffuser washing device consists of twodiffuser washing devices (2 a, 2 b), which are coupled in parallel withregard to the receipt and treatment of cooked pulp (20) obtained fromthe digester.
 6. Method according to claim 5, characterized in that thediffuser pump device consists of two diffuser pumps (P2 a, P2 b) whichare coupled in parallel and which are connected, on the inlet side, tothe first constituent flow (11) taken from the main feed line (10), withthe respective outlet from each of the diffuser pum devices onlysupplying one diffuser.
 7. Method according to claim 6, characterized inthat the wash filtrate (13 a, 13 b) which is obtained from eachrespective diffuser is firstly combined into a joint flow (16) in a Ycoupling, after which this combined wash filtrate flow is returned tothe main feed line (10) at the second point point (10 b) on the mainfeed line which is closer to the inlet of the digester pump (P3). 8.Method according to claim 1, characterized in that the system forsupplying the washing liquid to the digester and diffuser lacks anybuffer or intermediate storage tanks after the first feed pump (P1) haspressurized the washing liquid.